1. Field of the Invention
The present invention relates generally to sheet casting processes for producing continuous tapes or sheets of material from a liquid slurry, and more particularly to an improved level control system for maintaining the thickness of such tapes or sheets at a uniform, desired thickness.
2. Description of the Related Art
Continuous sheet casting processes are well known in the art for producing elongated tapes or sheets of material of a desired thickness. For example, it is well known to cast ceramic tape by depositing a liquid slurry, containing the ceramic material dissolved in a solvent, onto a moving belt, passing the belt under a “doctor” blade to limit the thickness of the slurry, and curing the tape by driving off the solvent in a heating step to leave an elongated tape of solid ceramic material. Such a ceramic tape might be used, for example, in the manufacture of interconnect substrates for micro-electronics.
In known sheet casting processes of this general type, the most common method of controlling the thickness of the tape is to control the height of the aforementioned “doctor” blade. A feedback control system has been used wherein the height of the tape is determined on the downstream side of the “doctor blade,” i.e., after the tape has passed beyond the “doctor” blade. In some systems, the tape height is determined just beyond the “doctor” blade when the tape is still “wet;” in other systems, the tape height is determined further downstream after the tape has cured. In either case, information relating to the tape height is used to control the height of the “doctor” blade above the moving belt in order to maintain the tape at a desired thickness. However, raising and lowering the height of the “doctor” blade only goes so far in producing a cast tape of uniform thickness.
Known casting systems often include valves for regulating the deposition of the slurry onto the moving belt of the casting system. The slurry is supplied from a pressurized source, or is pumped from an un-pressurized source, through a flexible tube. As the slurry can be abrasive and damaging to common types of control valves, the flow of slurry is typically controlled by a “pinch-off” control valve that extends about the flexible slurry supply tube. This pinch-off valve is typically either open (i.e., out of contact with the supply tube) or closed (i.e., it pinches off the supply tube). The slurry passing through the control valve is discharged into a reservoir located above the moving belt. In turn, the reservoir releases slurry onto the moving belt upstream from the “doctor” blade. It is known to include a sensor for sensing the slurry level in the reservoir to open or close the control valve in order to selectively discharge additional slurry into the reservoir. However, this process of regulating the level of slurry in the reservoir tends to be a discontinuous, start-and-stop process, which does not contribute to the production of a tape with uniform thickness. In addition, the application of excessive force to the supply tube during pinch-off (excessive crushing force) can prevent the supply tube from fully-opening when full-flow is later needed. Another problem that arises is that, depending on the type of tape being cast and the nature of the slurry, the diameter of the slurry supply tube that fills the slurry reservoir can vary, thereby changing the requirements of the pinch-off valve.
Moreover, solvent-based slurries pose a risk of explosion, as the solvent flames are often volatile. Known casting systems that sense the height of the tape downstream from the “doctor” blade use electronic sensors that are positioned in close proximity to the tape. However, such electronic sensors are subject to the creation of electrical sparks that, under some circumstances, could cause a dangerous and damaging explosion.
Accordingly, it is an object of the present invention to provide a level control system for a tape casting process capable of producing a casted tape with a thickness that is more uniform than tapes that are currently produced.
It is another object of the present invention to provide such a level control system that is relatively inexpensive and simple to construct.
Yet another object of the present invention is to provide such a level control system which reduces the risk of explosion of volatile solvent fumes while sensing the height of the tape being casted in real time.
A further object of the present invention is to provide such a level control system wherein the supply of slurry to the moving belt can be better regulated in a continuous manner as opposed to a start-and-stop manner.
These and other objects of the present invention will become more apparent to those skilled in the art as the description of the present invention proceeds.